ABSTRACT Background and aim The natural abundance of nitrogen isotopes (δ 15 N) provides a powerful integrative tracer of nitrogen uptake and assimilation in plants, yet the internal partitioning of δ 15 N among plant tissues remains insufficiently understood. Methods Here, we quantified isotopic discrimination during ammonium (NH 4 + ) and nitrate (NO 3 – ) uptake in maize ( Zea mays L.) grown under axenic hydroponic conditions at 0.2 and 2 mM N, representing low and high affinity transport systems, respectively. Results Distinct and systematic isotopic offsets between roots and shoots revealed contrasting assimilation patterns for the two nitrogen forms. Under NH 4 + nutrition, shoot and root δ 15 N were nearly identical, consistent with predominant assimilation in roots and limited translocation of reduced nitrogen. In contrast, NO 3 – nutrition produced strong 15 N enrichment in shoots relative to roots, reflecting spatially partitioned NO 3 – reduction and assimilation between organs. The δ 15 N of tissue NO 3 – was inversely related with its concentration, supporting isotopic fractionation during nitrate reduction and dilution by vacuolar storage. These results demonstrate that intra‐plant δ 15 N partitioning primarily reflects the spatial localization of nitrogen assimilation. Conclusion By establishing mechanistic baselines for intrinsic nitrogen isotope fractionation under controlled conditions, this study refines the physiological interpretation of plants δ 15 N and enhances its application to ecological and biogeochemical investigations.

The increasing prevalence of nitrate contamination in surface waters, groundwater, and ocean waters, represents a critical environmental challenge, particularly in regions with intensive agriculture and aquaculture. Denitrification, the microbial reduction of nitrate to dinitrogen gas, plays a pivotal role in mitigating this contamination and regulating the global nitrogen cycle. Stable isotope analysis provides critical insights into nitrate transformation pathways, distinguishing denitrification from anaerobic ammonium oxidation (anammox), another N-loss process, or internal recycling processes such as dissimilatory nitrate reduction to ammonium (DNRA).This review highlights the importance of isotopic tools for assessing nitrate attenuation in natural and anthropogenic-impacted systems and explores the use of nitrogen (δ15N) and oxygen (δ18O) isotopic fractionation to trace denitrification and to quantify its extent in diverse aquatic environments. The nitrogen (N) and oxygen (O) isotopic fractionation during denitrification is evaluated at organism and ecosystem levels. Also, environmental factors modulating isotopic composition of N compounds in groundwater, rivers, lakes, riparian zones, coastal wetlands and oxygen-deficient marine regions are explored.Advances in isotope biogeochemistry and analytical techniques improve our ability to assess the transport and fate of nitrate, integrating isotopic data with hydrological and biogeochemical models. A precise characterization of N and O isotopic enrichment factors for denitrification supports improved predictions of nitrogen cycling dynamics under changing environmental conditions. These approaches enhance understanding of nitrogen removal processes and help refine estimates of nitrogen fluxes at local, regional and global scales. By providing a quantitative framework for evaluating denitrification and related processes, this review contributes to developing more effective strategies for managing nitrogen pollution and mitigating its impacts on aquatic ecosystems.

Selenium plays a crucial role in estuarine biogeochemistry, balancing essential nutrient functions with potential environmental toxicity. This study examines the seasonal distribution of dissolved Se species, including volatiles, in the Adour estuary in relation to anthropogenic influences. To characterize major Se inputs from upstream watersheds to downstream tributaries, water samples were collected at low tide during three different seasons in upstream freshwaters, industrial/urban effluents and downstream estuarine waters. A tidal-cycle sampling campaign was conducted under low discharge conditions to assess Se dynamics during downstream estuarine mixing. Total dissolved Se (TDSe) concentrations ranged from 71 (pristine river) to 656 ng L-1 (industrial/urban-impacted tributaries). TDSe correlated strongly with nitrate (r = 0.84) in upstream waters, indicating significant agricultural and livestock contributions at the watershed scale. Selenate was the dominant species, followed by Se(-II+0) fraction and selenite. Volatile Se compound concentrations varied from 51 to 2757 pg L-1. Seasonal changes suggest that Se speciation is mainly controlled by watershed inputs derived from land use (agricultural and livestock practices) rather than downstream estuarine inputs. This speciation study further indicates that Se reactivity/bio-availability in estuarine systems can be largely influenced by anthropogenic activities, although further characterization of the aqueous reduced Se fraction is still needed.

Reactive nitrogen fate in natural systems remains difficult to predict because pathway partitioning occurs at the stage of nitrite turnover, where rapid and tightly coupled production and consumption processes obscure the underlying fluxes. Concentration-based assessments emphasize the dominant pools - nitrate and ammonium - while pathway divergence is determined at the stage of nitrite turnover, independently of pool size. Nitrite is the principal dissolved inorganic intermediate linking the oxidative and reductive branches of the nitrogen cycle and the obligatory precursor to all downstream dissolved and gaseous products. Because nitrite rarely accumulates, it has often been treated as a transient intermediate of limited interpretive value. This apparent invisibility reflects rapid, tightly coupled turnover and does not indicate functional insignificance. Its low and frequently undetectable concentration is the kinetic signature of this central position rather than evidence against it: rapid coupled turnover sustains high gross flux at near-zero standing concentration. Nitrogen retention, recycling and losses to the atmosphere are determined during nitrite turnover, where competing pathways partition fluxes according to kinetic and environmental constraints.Observed concentrations integrate formation and consumption into a net signal that masks opposing fluxes when internal cycling is rapid. Coupled delta N-15-delta O-18 measurements of nitrite constrain simultaneous production and consumption and differentiate biological from abiotic pathways. Partial oxygen isotope exchange with water increases the diagnostic primacy of delta 15N in resolving hidden turnover. However, its low concentration in natural environments can pose some challenges for analysis, requiring more sensitive approaches.Centering nitrogen-cycle interpretation on nitrite dynamics and isotopic expression across redox gradients from oxic soils to oxygen minimum zones, provides a mechanistic basis for predicting nitrogen budgets, N2O emissions, and ecosystem sensitivity to increasing redox variability under climate change and land-use intensification.

ABSTRACT Anthropogenic nitrogen inputs are increasing globally, while the conditions under which ecosystems transition from nitrogen retention to nitrogen loss remain poorly constrained. Environmental proxies commonly used to infer nitrogen loss, including redox status, carbon availability, and microbial gene inventories, provide limited power to predict when nitrate‐reduction pathways are expressed at the ecosystem scale. This limitation is particularly evident in coastal mangrove soils, where anaerobic and carbon‐rich conditions coexist with weak nitrate‐reduction activity. Here, we investigate nitrogen‐cycle organization along a spatially compressed mangrove–agricultural gradient on a Caribbean island by combining soil nitrogen pools, organic nitrogen isotopes (δ 15 N), inferred microbial metabolic functions putatively related to the microbial community (16S rDNA metabarcoding analysis), and controlled anoxic nitrate‐amended incubations. Across distances of less than 500 m, soils transitioned sharply from nitrogen‐rich, isotopically depleted mangrove sediments (total N up to 1.4%; δ 15 N≈2‰–3‰) to nitrogen‐poor, isotopically enriched agricultural soils (total N≈0.2%–0.6%; δ 15 N≈6‰–8‰). Despite widespread microbial functional potential associated with nitrate reduction throughout the transect, nitrate depletion under anoxic conditions remained slow in mangrove and back‐mangrove soils (0.56–1.15 nmol g −1 min −1 ) but was more than threefold faster in agricultural soils (2.32 nmol g −1 min −1 ). To integrate these long‐term isotopic and stoichiometric signals, we developed the Organic Nitrogen Index coupled to soil C/N ratios (ONI‐CN), an isotopic–stoichiometric framework combining δ 15 N and soil C/N ratios. Along the studied transects, ONI‐CN captured a sharp transition between two nitrogen‐cycle configurations, with mangrove and back‐mangrove soils clustering within a closed, recycling‐dominated state and agricultural soils in an open, loss‐dominated state, with no intermediate values. These observations support the interpretation that nitrate availability constrains the long‐term expression of nitrate‐loss pathways across the mangrove–agricultural gradient. The results are consistent with the presence of a functional nitrogen bank in mangrove soils, in which microbial nitrate reduction potential remains weakly expressed under chronically nitrate‐limited conditions. This framework highlights the sensitivity of nitrogen‐limited coastal ecosystems to increasing anthropogenic nitrate inputs and provides a conceptual basis for understanding threshold‐like reorganizations of nitrogen cycling under environmental changes.

The Amplex Red (AR) assay is a widely used method for the quantification of hydrogen peroxide (H2O2) in natural water due to its high sensitivity and specificity. However, the methodology developed for photochemistry may overestimate the H2O2 content estimated in sediment pore water, where H2O2 production results mainly from the oxidation of reduced species. Indeed, in previously published protocols, the measurement of H2O2 is not conducted a few minutes after the sample's mixing with the probe. We demonstrate that this delay is frequently sufficient for the reduced species present in pore water to produce a significant amount of H2O2. This study refines the AR technique by implementing high-frequency (HF, 10 Hz) measurements following probe addition. Moreover, the HF AR method gives access to three key parameters: I-the initial H2O2 concentration; II-the H2O2 production rate; and III-the total H2O2 production capacity of the sample upon oxidation. The accuracy and robustness of the method were demonstrated through a series of controlled experiments based on iron oxidation in the presence of citrate ligands. The efficiency of the method was demonstrated through its application in the field of sulfur- and iron-rich pore water. The findings represent a substantial enhancement in comparison to the prevailing protocols.

Understanding nitrogen (N) isotopic fractionation during plant uptake is critical for interpreting δ15N variations in terrestrial ecosystems. We investigated isotopic discrimination during ammonium (NH4 +) or nitrate (NO3 -) uptake in maize (Zea mays) grown hydroponically under controlled conditions with 0.2 and 2 mM to represent high and low affinity transport systems, respectively. Nitrogen (15ε) and oxygen (18ε) isotopic fractionation during NO3 - uptake were determined. NO3 - uptake exhibited low and concentration-independent 15ε values (0.2 mM: 15ε = -2‰; 2 mM: 15ε = -1.7‰). In contrast, 18ε was lower at high concentrations (0.2 mM: 18ε = -5.3‰; 2 mM: 18ε = -2.1‰). For NH4 + uptake, 15ε was higher and increased with concentration (0.2 mM: 15ε = -5.7‰; 2 mM: 15ε = -8.5‰). An isotope mixing model suggests a small NO3 - efflux contributes to 15N and 18O enrichment in solution due to significant isotopic fractionation during assimilation. The discrimination between source and plant δ15N is influenced by the source δ15N, the magnitude of 15ε, N supply, and uptake kinetics. While plant δ15N integrates source δ15N over time, it is unsuitable as a direct tracer. This study refines the understanding of isotopic fractionation mechanisms in plant N uptake and their implications for δ15N-based ecological investigations.

Galaxolide, a synthetic musk widely used in commercial products, including sanitation products and personal care products, poses environmental risks. Its sources and transformation pathways in aquatic environments remain poorly understood. Compound-specific isotope analysis (CSIA) offers a promising method to trace the origin and transformation of organic micropollutants by analyzing their individual isotopic signatures in natural abundance. While CSIA has been applied to various contaminants, its use with personal care products containing persistent organic pollutants is less explored. This study investigated the potential of CSIA for galaxolide to elucidate both abiotic and biotic transformations and trace its origin in personal care products. Batch experiments simulating photodegradation under UVC light and biodegradation by the strain Priestia sp. in MM20 medium revealed no significant isotopic fractionation (Delta C-13 < 0.5 parts per thousand). Despite the wide variation in the concentrations of galaxolide in personal care products, with some exceeding 15 g L-1, delta C-13 values alone did not allow discrimination between sources and origins. Further research is necessary to evaluate how various environmental processes, both abiotic and biotic, could affect the stable isotopic composition of galaxolide and other synthetic musks.

This study investigates isotopic fractionation associated with nitrate reduction in estuary sediments, focusing on the influence of nitrate and organic carbon availability. Flow-through reactor experiments using sediments from the Trieux, Seine, and Scheldt estuaries revealed that the isotopic enrichment factor (epsilon) varied widely, showing an inverse relationship with nitrate reduction rates only under nonlimiting nitrate conditions. Under these conditions, increasing reduction rates diminished enzyme selectivity, leading to lower isotopic discrimination, while at lower rates, isotopic enrichment increased due to substrate selectivity. The addition of carbon further enhanced reduction rates, confirming the carbon limitation as a key driver of isotopic fractionation patterns. These results highlight the importance of substrate availability in shaping isotopic signatures in natural sediments beyond reduction rates alone. Our findings provide critical insights for interpreting nitrate isotopic data in field studies, emphasizing the need to consider local substrate dynamics for accurate assessments of nitrogen cycling in coastal ecosystems.

The measurement of 87Sr/86Sr isotope ratios is a powerful tool for determining the geographical origin of food products, helping to fight against food fraud. This study evaluates the potential of Sr isotopic ratios for tracing the provenance of Tunisian olive oil. Soil, roots, leaves, olive oil, and pomace were analyzed from multiple orchards across Tunisia. A strong correlation (r = 0.99, p < 0.001) between the 87Sr/86Sr isotope ratio of olive oil and the paired soil confirms the geological influence. Statistical analysis (ANOVA, p > 0.05) demonstrated that extraction methods for the production of olive oil do not significantly alter the Sr isotopic signature. These findings establish Sr isotope analysis as a robust, processing-independent geographical tracer. By developing a Sr isotope database for Tunisian olive oil, this study provides a scientific framework for authentication, supporting regulatory efforts to prevent mislabeling and protect product integrity in global markets.

Sulfidic hot springs harbor unique microbial communities and are important in mercury (Hg) species transformations, although the fine scale drivers of these processes remain poorly understood. Here we studied Hg speciation in water, biofilms, and sediment across three sampling seasons in a French sulfidic hot spring with low Hg concentrations. Microbial Hg species methylation and demethylation potentials were evaluated using incubation experiments with species-specific Hg isotope tracers. Temporal variation in inorganic Hg (iHg) and methylmercury (MeHg) concentrations in water, biofilm, and sediment was observed. The incubation of microbial communities in biofilms and sediment under dark conditions exhibited low iHg methylation potentials, whereas a significant extent of biotic MeHg demethylation to oxidized iHg was found in relation to MeHg concentrations. Results from microbial diversity (16S rDNA) and the metabolic inhibition experiments suggest an important role of sulfur-linked bacterial metabolism dynamics. Specifically, sulfate-reducers and anoxygenic phototrophs were important factors in the regulation of MeHg concentrations in our study site. Overall, the observed dominance of microbial MeHg demethylation demonstrates a strong Hg detoxification capacity in sulfidic aquatic environments.

Intensified agricultural practices, particularly the increased use of nitrogen fertilizers, are fueling the rise of nitrous oxide (N2O) in groundwater. This gas is produced through the nitrification/denitrification of inorganic nitrogen in the groundwater. Based on combined chemical and isotopic analyses of nitrate (NO3-) and chemical analysis of N2O, we present the first-ever evidence for widespread denitrification-driven N2O accumulation in Indian groundwater that often exceeds 25 times the saturation level. This phenomenon is particularly concerning in regions where low precipitation is received with intensive groundwater irrigation, leading to an inadvertent accumulation of N2O in the groundwater. The emission factor for groundwater (EF5g) from the agricultural soils of India is significantly lower (0.00067) than the global mean values (0.008), suggesting that indirect fluxes of N2O from the Indian soils are lower than the global mean. Implementing sustainable agricultural practices and maximum feasible control measures could further reduce indirect N2O emissions from agricultural soils and their associated environmental consequences.

Contaminants of Emerging Concern (CECs) are human-made chemicals that remain unregulated. The continuous detection of CECs in aquatic ecosystems, due to their incomplete removal, emphasizes the importance of understanding their fate and impact on the environment and human health. The detrimental effects of CECs on marine eukaryotes are well documented in multiple studies. However, their impact on marine bacteria and their biodegradation by these organisms are not well understood. In this study, two marine bacteria, Priestia sp. 35 ODPABA G14 and Rhodococcus sp. 23 AHTN G14, previously isolated from submarine sediments, were used. These two strains were tested for their resistance as well as their capacity to degrade different classes of hydrophobic and hydrophilic CECs, including synthetic musks, UV filters, pesticides and pharmaceuticals. Both strains showed high resistance to all of the hydrophobic tested CECs even up to 500 mg L−1. Only Ketoprofen was toxic to bacterial cells, particularly to Rhodococcus sp. starting at concentration as low as 4 mg L−1. Furthermore, Priestia sp. and Rhodococcus sp. strains exhibited high biodegradation potential, especially for hydrophobic compounds. Although this may not apply to all pollutants, the data presented in this study suggest a positive correlation between marine bacterial resistance to CECs and their high biodegradation potentials.

Galaxolide (HHCB) is the most common synthetic musk compound detected in numerous daily products. Despite its persistence in the aquatic environment, the photodegradation of HHCB remains poorly understood. In this study, we investigated the direct and indirect photolysis kinetics of HHCB under simulated sunlight and UVC light. Our aim was to determine the role of reactive oxygen species (ROS) responsible for HHCB degradation in the aquatic environment and to identify its transformation products. The influence of environmental factors on indirect photolysis was investigated by testing both synthetic waters (containing humic acid, carbonate (CO32-), and nitrate (NO3-)) and real waters (riverine and effluent). Hydrogen peroxide (H2O2/UVC) was tested to simulate the wastewater treatment process. Quencher experiments were conducted to identify the role of ROS in HHCB photodegradation, including hydroxyl radicals ((OH)-O-center dot), carbonate radicals (CO3(center dot)-), triplet states of dissolved organic matter (3DOM*), and singlet oxygen (1O2). The results clearly indicated that HHCB was efficiently degraded by direct photolysis under both light conditions. The presence of H2O2 led to the most efficient HHCB degradation due to the high production of (OH)-O-center dot induced under UVC. Indirect photolysis contribution was observed, induced by (OH)-O-center dot, CO3(center dot)-, 3DOM*, and 1O2 to different extents depending on the light and matrix composition. The experiments led to the detection of transformation products: HHCB lactone, a well-known transformation product, and two other substances with proposed structures. This study provides a comprehensive identification of the processes involved in the direct and indirect photodegradation of HHCB, which could serve as the basis for evaluating and modeling the fate of HHCB in aquatic environments. Galaxolide (HHCB) is the most common synthetic musk compound detected in numerous daily products.

The interconnection between biotic and abiotic pathways involving the nitrogen and iron biogeochemical cycles has recently gained interest. While lacustrine ecosystems are considered prone to the biotic nitrate reduction (denitrification), their potential for promoting the abiotic nitrite reduction (chemodenitrification) remains unclear. In the present study, batch incubations were performed to assess the potential for chemodenitrification and denitrification in the saline inland lake Gallocanta. Sulfidic conditions are found in top sediments of the system while below (5-9 cm), it presents low organic carbon and high sulfate and ferrous iron availability. Anoxic incubations of sediment (5-9 cm) and water from the lake with nitrite revealed potential for chemodenitrification, especially when external ferrous iron was added. The obtained isotopic fractionation values for nitrite (ɛ15NNO2) were -6.8 and -12.3 ‰ and therefore, fell in the range of those previously reported for the nitrite reduction. The more pronounced ɛ15NNO2 (-12.3 ‰) measured in the experiment containing additional ferrous iron was attributed to a higher contribution of the chemodenitrification over biotic denitrification. Incubations containing nitrate also confirmed the potential for denitrification under autotrophic conditions (low organic carbon, high ferrous iron). Higher reaction rate constants were found in the experiment containing 100 μM compared to 400 μM nitrate. The obtained ɛ15NNO3 values (-8.5 and -15.1 ‰) during nitrate consumption fell in the range of those expected for the denitrification. A more pronounced ɛ15NNO3 (-15.1 ‰) was determined in the experiment presenting a lower reaction rate constant (400 μM nitrate). Therefore, in Gallocanta lake, nitrite generated during nitrate reduction can be further reduced by both the abiotic and biotic pathways. These findings establish the significance of chemodenitrification in lacustrine systems and support further exploration in aquatic environments with different levels of C, N, S, and Fe. This might be especially useful in predicting nitrous oxide emissions in natural ecosystems.

Nitrate is accumulated in the groundwater, modified through nitrification/denitrification, and exchanged with coastal/estuarine water bodies. To examine the sources and modifications of nitrate, the concentrations and isotopic composition of nitrate (delta 15N and delta 18ONO3) in the groundwater was monitored at 5 locations along the bank of Godavari estuary and in the estuarine waters for 7 months during wet (August-November) and dry (March-May) periods. Though the concentration of nitrate (NO3-) was higher during the wet than dry periods in both the groundwater and estuary, insignificant seasonal variability was observed in delta15N and delta18ONO3 indicating homogenization through mineralization-immobilization turnover of NO3-. The range of delta15N to delta18O of NO3- indicates soil, manure, and septic waste may be the major source of NO3-. The mean ratio of delta15N/delta18O of 1.1 +/- 0.3 indicates the occurrence of denitrification in the groundwater. Concentrations delta15NNO3 and delta18ONO3 of NO3- displayed a significant relation between groundwater and estuarine water suggesting that NO3- is possibly denitrified. This study suggests that denitrified NO3- (enriched delta15N and delta18O) reported in the Godavari estuary may be contributed through submarine groundwater discharge than it is hypothesized to flux from the watershed.

The relationship between delta O-18 and delta N-15 in aquatic nitrate (NO3-) is used to assess nitrogen (N) cycling, primarily relying on controlled laboratory tests of isotope fractionation from nitrification and denitrification. Nevertheless, laboratory findings frequently contradict the evolution of the nitrate delta O-18/delta N-15 ratios observed in natural river systems. We investigated this disparity by using moderated regression modeling, analyzing a global data set (n = 1303) of nitrate isotopes encompassing rivers with varying NH4+/NO3- ratios and delta O-18-H2O values. First, our analysis revealed that elevated delta O-18/delta N-15 ratios (>0.6) were prevalent in rivers with high NH4+/NO3- ratios, suggesting reducing conditions that could potentially promote denitrification and/or ammonium accumulation. By contrast, lower delta O-18/delta N-15 ratios (<0.5) predominated in rivers with low NH4+/NO3- conditions, suggesting oxidizing conditions favoring increased NH4+ removal through nitrification. Second, when delta O-18-H2O values were low, it resulted in reduced delta O-18-NO3- values during nitrification, which in turn lowered the delta O-18/delta N-15 ratios. We discovered that the delta O-18/delta N-15 ratios in nitrate were elevated in the fall, likely due to predominant processes, such as denitrification, and lower in the winter due to lower delta O-18-H2O values. This global river assessment suggests a more significant influence of ammonium and the role of water oxygen in riverine N-nutrient isotope cycling than was previously considered.

The sulphur cycle has a key role on the fate of nutrients through its several interconnected reactions. Although sulphur cycling in aquatic ecosystems has been thoroughly studied since the early 70’s, its characterisation in saline endorheic lakes still deserves further exploration. Gallocanta Lake (NE Spain) is an ephemeral saline inland lake whose main sulphate source is found on the lake bed minerals and leads to dissolved sulphate concentrations higher than those of seawater. An integrative study including geochemical and isotopic characterization of surface water, porewater and sediment has been performed to address how sulphur cycling is constrained by the geological background. In freshwater and marine environments, sulphate concentration decreases with depth are commonly associated with bacterial sulphate reduction (BSR). However, in Gallocanta Lake sulphate concentrations in porewater increase from 60 mM at the water–sediment interface to 230 mM at 25 cm depth. This extreme increase could be caused by dissolution of the sulphate rich mineral epsomite (MgSO 4 ·7H 2 O). Sulphur isotopic data was used to validate this hypothesis and demonstrate the occurrence of BSR near the water–sediment interface. This dynamic prevents methane production and release from the anoxic sediment, which is advantageous in the current context of global warming. These results underline that geological context should be considered in future biogeochemical studies of inland lakes with higher potential availability of electron acceptors in the lake bed compared to the water column.

Mangrove sediments are known to be potentially active reducing zones for nitrogen removal. The goal of this work was to investigate the potential for nitrate reduction in marine mangrove sediments along a canal impacted by anthropogenic activity (Guadeloupe, Lesser Antilles). To this end, the effect of nitrate concentration, organic carbon load, and hydraulic retention time was assessed as factors affecting these nitrate reduction rates. Nitrate reduction potential was determined using flow-through reactors in marine mangrove sediments collected along "The Canal des Rotours" in Guadeloupe. Potential nitrate reduction rates, in the presence of indigenous organic carbon, generally increased upon increasing nitrate supply from around 120 nmol cm-3 h-1 (low nitrate) up to 378 nmol cm-3 h-1 (high nitrate). The potential for nitrate reduction increased significantly with the addition of mangrove leaves, whereas the addition of simple, easily degradable carbon (acetate) resulted in an almost fivefold increase in nitrate reduction rates (up to 748 nmol cm-3  h-1 ). The hydraulic retention time also had an impact on the nitrate reducing capacity due to an increased contact time between nitrate and the benthic microbial community. Marine mangrove sediments have a high potential to mitigate nitrogen pollution, mainly governed by the presence of large amounts of degradable carbon in the form of litter. The mangrove sediments from this Caribbean island, currently exposed to a small tidal effect, could increase their nitrate elimination capacities due to prolonged water retention via engineering.